A wireless communication is extensively developed to provide various types of communication services such as voice, data, and the like. In general, a wireless communication system is a multiple access system supporting communication with multiple users by sharing available system resources (bandwidth, transmission power, etc.). The multiple access system includes a CDMA (code division multiple access) system, an FDMA (frequency division multiple access) system, a TDMA (time division multiple access) system, an OFDMA (orthogonal frequency division multiple access) system, an SC-FDMA (single carrier frequency division multiple access) system, and the like.
The 2nd-generation mobile communication refers to transmission/reception voice data in a digital manner, which includes CDMA, GSM, and the like. As an advanced level of GSM, GPRS has been proposed to provide a packet switched data service based on the GSM system.
The 3rd-generation mobile communication refers to a scheme for transmitting and receiving image and data. A Third Generation Partnership Project (3GPP) has developed a mobile communication system (IMT-6000) and adopts WCDMA as a radio access technology (RAT). A scheme combining the IMT-6000 technology and the RAT, e.g., the WCDMA, is called universal mobile telecommunication system (UMTS). A UTRAN stands for a UMTS terrestrial radio access network.
The 3rd-generation mobile communication is evolving into 4th-generation mobile communication.
As the 4th-generation mobile communication technology, a long-term evolution network (LTE) technique under standardization by 3GPP and an IEEE 802.16 technique under standardization by IEEE have been proposed. The LTE uses a term of an evolved-UTRAN (E-UTRAN).
For the 4th-generation mobile communication technology, an orthogonal frequency division multiplexing (OFDM)/orthogonal frequency division multiple access (OFDMA) have been introduced. OFDM uses orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). A transmitter performs IFFT on data and transmits the same. A receiver performs FFT on a reception signal to restore the original data. The transmitter uses IFFT to combine a plurality of sub-carriers, and the receiver uses corresponding FFT to separate the plurality of sub-carriers.
Meanwhile, in the 3rd or 4th-generation mobile communication system, attempts for increasing a cell capacity continues to support high capacity services such as multimedia contents, streaming, and the like, and bi-directional services.
Namely, as various large capacity transmission techniques are required in line with the development of communications and the spread of multimedia technology, a method of allocating more frequency resources may be employed to increase radio capacity, but allocation of more frequency resources to multiple users with limited frequency resources has a limitation.
An approach of using a high frequency band and reducing a cell radius has been proposed to increase the cell capacity. The application of a cell with a relatively small cell radius such as a pico cell or the like can use a higher band than the frequency band used in the existing cellular system, having an advantage that more information can be transmitted, but also there is shortcomings in that more base stations should be installed in the same area, incurring more costs.
As one of approaches for increasing the cell capacity by using smaller cells, a femtocell has been proposed.
A femtocell refers to providing a small radio environment by installing a very small base station with low power consumption at homes or offices. The femtocell can enhance quality of service (QoS) by improving an indoor service available area and increasing capacity, and is expected to completely settle the next generation mobile communication system by providing data services.
Thus, research on the technique for a femto base station (BS) installed in a residential area or in offices is actively ongoing. A femto BS refers to a very small mobile communication base station used at homes or offices. The femto BS is connected to an internet protocol (IP) network provided at homes or offices and accesses a core network of a wireless communication system via the IP network to provide a radio communication service. A user may receive a service via an existing macro BS at outside of a room, and receive a service via the femto BS in a room. The femto BS improves an indoor coverage of the wireless communication by complementing the fact that the service of the existing macro BS is aggravated in a building. In addition, because the femto BS provides a service only to predetermined particular users, it can provide a high quality voice service and data service.
A method for allocating a frequency band of the femto BS disposed within a coverage of the macro BS includes a co-channel scheme, a partial co-channel scheme and a dedicated channel scheme. The co-channel scheme is allocating the same frequency band as that of the macro BS, as a frequency band of the femto BS. In this case, it is important to control transmission power of the femto BS to reduce interference that may be possibly generated as the macro BS and the femto BS use the same frequency band. The partial co-channel scheme is allocating a partial frequency band of the macro BS as a co-channel which is to be used together with the femto BS. In this scheme, if interference occurs in the co-channel, a user of the macro BS may receive a service via a frequency band other than the co-channel, thus reducing the interference. The dedicated-channel scheme is allowing the macro BS and the femto BS use different frequency bands, significantly reducing interference between the macro BS and the femto BS.
The femto BS is a small base station that a user purchases and installs it in his office or home, so a radio communication provider cannot restrain disposition of the femto base station. In general, distributions of femto BSs is not regular and unpredictable. Thus, a method for reducing interference between arbitrarily disposed femto BSs is required.
In order to reduce interference between femto BSs, transmission power of femto BSs may be limited, but his method has a problem in that the coverage of each femto BS is reduced. Using of different frequency bands between adjacent femto BSs can significantly reduce interference between the femto BSs and the coverage of each femto BS can be guaranteed. Namely, if each femto BS uses a different frequency band and uses a maximum frequency resource, each femto BS could use a high frequency efficiency.
Thus, a method for reducing interference between femto BSs and improving a frequency efficiency is required.
In this respect, a reduction in the cell radius may lead to dense base stations in a particular area, and because of unnecessary overlap installations of base stations, interference is generated, radio resources are ineffectively used, and a system performance is degraded.
In particular, in the femtocell cellular system, a femto BS can be arbitrarily installed by a user, and such installation of femto BSs possibly causes an overcrowded femto BSs.
In the related art as described above, an installation position of a femto BS is selected by each user, and the femto BS has a smaller coverage compared with a macro-base station and provides a service to a smaller number of determined users.
Thus, the location distribution of users much affect the interference relationship between femto BSs. Unnecessary overlap installations and overcrowded installations may be caused, and in a residual area or offices are crowded, the distance between installed femto BSs is narrow.